Attempts will be made to induce leukemic transformation in multipotential murine hemopoietic stem cells able to be cloned in semisolid agar cultures. Target cell populations will include FACS-enriched progenitor cells from CBA fetal liver and adult marrow cells from mice pretreated with 5FU and endotoxin. Attempts will be continued to purify the factor stimulating mixed hemopoietic colony formation in vitro from pokeweed mitogen-stimulated spleen conditioned medium (SCM) and to establish whether the same factor is involved in growth regulation of the SCM-dependent Friend virus-induced leukemias able to be cloned in vitro. T-cell hybridomas will be further explored as a source material for hemopoietic regulator production. Characterization will continue of the differentiation-enforcing factor in post-endotoxin mouse serum able to induce differentiation in the myelomonocytic leukemia, WEHI-3 and to suppress stem cell self-replication in this leukemia. Attempts will be made to determine the site of production of this factor in vivo and to develop methods for mass producing the factor in vitro.